Tris-aryl-o-hydroxyphenyl-s-triazines are intermediates for or are themselves UV absorbers useful for the protection of natural or synthetic materials from the adverse action of actinic radiation. There are many methods described for the preparation of such s-triazines as seen from the publications of S. Tanimoto and M. Yamagata, Senryo to Yakuhim, 1995, 40(12), 325-339 and of H. Brunetti and G. E. Luethi, Helv. Chim. Acta, 1972, 55(5), 1566-1595.
The most versatile method is to employ one or more Friedel-Crafts reactions starting from cyanuric chloride. A major obstacle in this approach is the fact that Friedel-Crafts reactions of aryl groups and cyanuric chloride are non-selective. This is a problem when the goal is to prepare an asymmetric tris-aryl-s-triazine. This is explained in the Tanimoto and Brunetti publications mentioned above and in U.S. Pat. Nos. 5,084,570 and 5,106,972. However, it is well-known that substitution reactions between nucleophiles and cyanuric chloride are selective as taught by E. M. Smolin and L. Rapoport, s-Triazines and Derivatives in The Chemistry of Heterocyclic Compounds, A. Weissberger Ed., Interscience Publishers, New York, 1959, pp. 53-57.
One possible approach to prepare asymmetric tris-aryl-s-triazines selectively would be to (a) replace one or two chlorine atoms of cyanuric chloride with an appropriate protecting group in a nucleophilic manner, (b) replace the remaining chlorine atom(s) with the desired aryl group via a Friedel-Crafts reaction; (c) displace the protecting group(s) with chlorine, and finally (d) replace the newly formed chlorine(s) with the second aryl group in a Friedel-Crafts manner. U.S. Pat. Nos. 5,084,570 and 5,106,972 disclose this strategy for the preparation of 2,(2,4-dihydroxyphenyl)-4,6-diaryl-s-triazine. The protecting group chosen in these two patents is methyl mercaptan. The process outlined is four steps starting from cyanuric chloride.
Another drawback of Friedel-Crafts reactions of aryl groups and cyanuric chloride in addition of non-selectivity is the fact that large amounts of Lewis acid are necessary to mediate the reaction, normally equimolar amounts. The Lewis acid most commonly used is aluminum chloride. These reactions produce prodigious amounts of aluminum waste which is environmentally hard to handle.
Japanese Hei 9-59263 discloses a three-step synthetic approach for the preparation of asymmetric tris-aryl-hydroxyphenyl-s-triazines. The preferred method of the Japanese reference is a one-pot process using a Lewis acid to mediate all three steps. This reference will be discussed in more detail later in this application.
A welcome addition to the art therefore would be to (a) provide a method of performing Friedel-Crafts reactions between aryl groups and s-triazines using protic acids instead of Lewis acids, and to (b) provide a method of preparing asymmetric tris-aryl-s-triazines in less than four synthetic steps.